The high costs associated with several innovative cancer therapies, most notably monoclonal antibodies (mAbs), have raised a concern that even among cancer patients with insurance, mAbs may be financially unaffordable due to the associated high out-of-pocket payment. Such high costs create a vulnerable group of cancer patients known as the under-insured. The objectives of our study are to use claims data to examine the diffusion of innovative monoclonal antibody agents among cancer patients younger than 65 years who have private insurance; to explore whether underinsurance may constitute a barrier to the access of these novel therapies; and to determine their impact on the costs of cancer care. The study has five specific aims: (1) to describe the trend of mAb use among non-elderly cancer patients with private insurance; (2) to examine whether mAb utilization patterns differ by patient characteristics and/or insurance generosity in the types of cancer with approved indication for mAb use; (3) to estimate the impact of mAbs on the costs of cancer care and cost increases over time in the types of cancer with approved indication for mAb use; (4) to determine factors associated with off-label mAb use; (5) to estimate the overall costs of cancer care and cost increases over time and to ascertain the associated cost drivers. For each study aim, appropriate econometric methods will be applied to analyze factors associated with mAb use, and cancer treatment costs. Separate analyses will be performed for all cancers, and for each of the three cancer sites that are most likely to be treated with mAbs: non-Hodgkin's lymphoma, colorectal cancer, and breast cancer. As immunotherapy, in particular, mAbs, becomes increasingly important in cancer care, this study will inform policymakers of the diffusion of recent therapeutic innovations among cancer patients and the impact of innovative agents such as mAbs on the costs of cancer care. This information will help project the future costs of cancer. Findings from this study will also alert policymakers to the subgroups of patients who may not benefit from innovative agents such as mAbs due to access barriers.